It is quite another matter – rombovy jackscrew. It was such jacks are equipped with almost all modern cars. He acts ridiculously easy – in fact it's clamp, compressing two hinged arm. Kink and the corresponding opposite movement of the pair of shoulders with each other and is actually useful action of the jack. Compact, low self-weight, ease of operation and last but not least, the cheapness of the place identified such jacks in the trunk of cars.
Moreover, many models have a special Profile spare wheel recess for laying and fixing it rombovogo jack. Another advantage of this design – a small pick-up height, you have a car with low clearance and punctured a wheel can be raised in such a jack without any problems. There are also disadvantages. First and foremost – the presence of friction force joints. Let me explain – hinge, moving along the screw and its thread and thread of the screw exposed to considerable mechanical stress. Degree fitting a pair of hinge screw needs to be very precise to avoid skewing and jamming, which in turn makes this pair of extremely sensitive to foreign particles and objects, as well as to the presence of lubricant. The second disadvantage – small lift the weight. Staff jack often used for lifting cargo weighing up to a half tons, the weight is greater than this can frustrate a screw or threaded joint. If you add the low (30-40%) efficiency and a significant effort to handle, it becomes clear – the heavy weight rombovy jack not a friend.